If you have a process ID but aren't sure whether it's valid, you can use the most unlikely of candidates to test it: the kill command. If you don't see any reference to this on the kill(1) man page, check the info pages. The man/info page states that signal 0 is special and that the exit code from kill tells whether a signal could be sent to the specified process (or processes).

So kill -0 will not terminate the process, and the return status can be used to determine whether a process is running. For example:

 $ echo $$     # show our process id
 12833
 $ /bin/bash   # create new process
 $ echo $$     # show new process id
 12902
 $ kill -0 12902
 $ echo $?     # exists, exit code is 0
 0
 $ exit        # return to previous shell
 $ kill -0 12902
 bash: kill: (12902) - No such process
 $ echo $?     # doesn't exist, exit code is 1
 1

Many UNIX dæmons store their process IDs in a file in /var/run when they are started. Using kill -0 to test the pid is a lot easier than parsing ps output. For example, to test whether cron is running, do the following:

 # kill -0 $(cat /var/run/cron.pid)
 # echo $?
 0